The present invention relates generally to a method for making nanoporous silicone resins which are useful for forming low dielectric constant films. More specifically, the present invention in a preferred embodiment is a method for making a nanoporous silicone resin having a narrow pore size range by hydrosilating a hydridosilicon containing resin with a 1-alkene comprising about 8 to 28 carbon atoms to form an alkylhydridosiloxane resin, coating the alkylhydridosiloxane resin on a substrate, and heating the coated substrate to effect thermolysis of alkyl substituents comprising about 8 to 28 carbon atoms of the alkylhydridosiloxane resin to form a nanoporous silicone resin.
Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements forming an integrated circuit (IC). These interconnect levels are typically separated by an insulating or dielectric film. Previously, a silicon oxide film formed using chemical vapor deposition (CVD) or plasma enhanced techniques (PECVD) was the most commonly used material for such dielectric films. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide films is inadequate to provide adequate electrical insulation.
In order to provide a lower dielectric constant than that of silicon oxide, dielectric films formed from siloxane-based resins have found use. An example of such films are those formed from poly(hydrogen)silsesquioxane resins as described for example in Collins et al., U.S. Pat. No 3,615,272 and Haluska et al. U.S. Pat. No. 4,756,977. While such films provide lower dielectric constants than CVD or PECVD silicon oxide films and also provide other benefits such as enhanced gap filling and surface planarization, typically the dielectric constants of such films are limited to approximately 3 or greater.
It is well known that the dielectric constant of the above discussed insulating films is an important factor where IC's with low power consumption, cross-talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials and methods for making such materials that can provide electrically insulating films with dielectric constants below 3 are desirable. In addition it is desirable to have siloxane-based resins and method for making such resins that provide low dielectric constant films which have a high resistance to cracking. Also, it is desirable for such siloxane-based resins to provide low dielectric constant films by standard processing techniques.
It is known that the dielectric constant of solid films decrease with a decrease in density of the film material. Therefore considerable work is being conducted to develop porous insulating films for use on semiconductor devices.
Kapoor, U.S. Pat. No. 5,494,859, describes a low dielectric constant insulating layer for an integrated circuit structure and a method of making the layer. A porous layer is formed by depositing on a structure a composite layer comprising an insulating matrix material and a material which can be converted to a gas upon subjection to a converting process. Release of the gas leaves behind a porous matrix of the insulating material which has a lower dielectric constant than the composite layer. The matrix forming material is typically silicon oxide and the material which can be converted to a gas upon subjection to a converting process is exemplified by carbon.
Hedrick et al., U.S. Pat. No. 5,776,990, describe an insulating foamed polymer having a pore size less than about 100 nm made from a copolymer comprising a matrix polymer and a thermally decomposable polymer by heating the copolymer above the decomposition temperature of the decomposable polymer. The copolymers described are organic polymers that do not contain silicon atoms.
Smith et al., WO 98/49721, describe a process for forming a nanoporous dielectric coating on a substrate. The process comprises the steps of blending an alkoxysilane with a solvent composition and optional water; depositing the mixture onto a substrate while evaporating at least a portion of the solvent; placing the substrate in a sealed chamber and evacuating the chamber to a pressure below atmospheric pressure; exposing the substrate to water vapor at a pressure below atmospheric pressure and then exposing the substrate to base vapor.
Mikoshiba et al., Japanese Laid-Open Patent (HEI) 10-287746, describe the preparation of porous films from siloxane-based resins having organic substituents which are oxidized at a temperature of 250.degree. C. or higher. The useful organic substituents which can be oxidized at a temperature of 250.degree. C. or higher given in this document include substituted and unsubstituted groups as exemplified by 3,3,3-trifluoropropyl,.beta.-phenethyl group, t-butyl group, 2-cyanoethyl group, benzyl group, and vinyl group.
Mikoskiba et al., J Mat. Chem., 1999, 9, 591-598, report a method to fabricate angstrom size pores in poly(methylsilsesquioxane)films in order to decrease the density and the dielectric constant of the films. Copolymers bearing methyl(trisiloxysilyl) units and alkyl(trisiloxysilyl) units are spin-coated on to a substrate and heated at 250.degree. C. to provide rigid siloxane matrices. The films are then heated at 450.degree. C. to 500.degree. C. to remove thermally labile groups and holes are left corresponding to the size of the substituents. Trifluoropropyl, cyanoethyl, phenylethyl, and propyl groups were investigated as the thermally labile substituents.
Hacker et al., WO 98/47941 Publication, describes a process for preparing organohydridosiloxane resins.
Hacker et al., WO 98/47944 Publication "'944 Pub.", describes an organohydridosiloxane polymer having a cage conformation, at least approximately 40 mole percent carbon containing substituents and a dielectric constant of less than about 2.7. Hacker et al., '944 Pub., teach the organic substituent of the organohydridosiloxane can be a normal or branched alkyl comprising 1 to 20 carbon atoms. In the examples Hacker et al, '944 Pub., demonstrate that the organic substituent can be methyl, phenyl, benzyl, tert-butyl, and chloromethyl and that organohydridosiloxane polymers containing such substituents can be heated at temperatures ranging from 380.degree. C. to 450.degree. C. to provide cured resins having dielectric constants ranging from 2.43 to 2.66.
Hacker et al., WO 98/47945 Publication "'945 Pub.", teach organohydridosiloxane polymer having between approximately 0.1 to 40 mole percent carbon-containing substituents, and a dielectric constant of less than about 3.0. Hacker et al., '945 Pub., teach that the organic substituent of the polymer can be for example normal and branched alkyls. In the examples, Hacker et al., '945 Pub., demonstrate that the organic substituents can be methyl, ethyl, propyl, n-butyl, cyclohexyl, phenyl, t-butyl, and trifluoropropyl. Hacker et al., '945 Pub., demonstrate that organohydridosiloxane containing the described organic substituents can be cured by heating at temperatures ranging from 380.degree. C. to 425.degree. C., and that such cured materials can have a dielectric constant ranging from 2.82 to 3.09.
It is clear from the examples provided in the above referenced Hacker et al. documents that Hacker et al. did not appreciate the improvement in dielectric constants that could be achieved by hydrosilating hydridosiloxane resins with 1-alkenes comprising about 8 to 28 carbon atoms to form alkylhydridosiloxane resins and subsequently curing the resin and thermolysing the alkyl substituents comprising about 8 to 28 carbon atoms of the alkylhydridosiloxane resins to form nanoporous siloxane resins. Generally, the organic substituents of the organohydridosiloxane resins used in the examples of Hacker et al. were too small to provide the amount of porosity required for optimal dielectric constants for the cured resins.